1. Field of the Invention
The present invention generally relates to the formation of images with toner. More particularly, the present invention relates to an apparatus and a method to provide a UV-curable coating on toner images formed on a web.
2. Description of the Related Technology
The formation of toner images on a receptor material by means of an image reproduction system such as a printing or copying system is well known. In so-called direct image reproduction systems, such as direct electrostatic printing (DEP) systems, the toner is image-wise deposited on an image-receiving member. This image-receiving member can be the receptor material or an intermediate transfer member. In the latter case, the toner images are subsequently transferred to the receptor material. However, nowadays, in most image reproduction systems based on typical graphical processes including amongst others electrophotography, ionography and magnetography, instead of direct printing a latent image is formed corresponding to either the original to be copied or to digitized data describing an electronically available image. In electrophotography for instance, which is currently the most widespread, a charged latent image is formed on a pre-charged photosensitive member by image-wise exposure to light. This latent image is subsequently made visible on the image-forming member with developer at a development zone, the developer comprising, or consisting of, charged toner. The toner particles may constitute dry particulate matter. Alternatively, a wet liquid type developer may be used wherein the toner particles are dispersed in a solvent. In systems employing dry toner particles as developer, the development may be carried out by different methods as for instance “cascade,” “magnetic brush,” “powder cloud,” “impression” or “transfer” development. After the development of the latent image, the developed image is transferred to a receptor material, directly or via one or more intermediate image-carrying members, where it may be permanently fused.
The toner images fused to the receptor material are to a certain extent vulnerable to physical interaction and can be e.g. scratched or otherwise damaged. Especially composite and/or multi-layer images, such as e.g. full color images are vulnerable due to the increased topography, e.g., the height differences within the toner image with respect to the surface of receptor material carrying the toner images. Besides the possible shortcomings of toner images fused on a receptor material such as e.g. lack of mechanical strength, unsatisfactory resistance to wear, and the possible negative impact over time of UV irradiation on the color rendering, there is also the look and feel of the toner images. Particularly for the reproduction of recorded images, such as e.g. photos, still images, greeting cards, covers, etc., the customer expects the look and feel of images produced by offset printing with appropriate finishing, where the printed images have a high brilliance, a smooth surface (e.g., without topography), an even gloss distribution and often a high gloss.
Protective overcoats on toner images and processes entailing the overcoating of toner images are known. For instance, U.S. Pat. No. 4,477,548 (assigned to Eastman Kodak Co) discloses radiation-curable coating compositions which can be used to provide protective overcoat layers. The overcoat layer is formed by coating a curable coating composition onto the toner image bearing substrate and curing the resulting coating to bond it to the substrate. The toner images may be formed by electrography. The curable coating composition comprises (a) either (i) a mixture of a siloxy-containing polycarbonol and an acrylated urethane or (ii) a siloxy-containing acrylated urethane, (b) a multifunctional acrylate, and, optionally (c) a free radical photoinitiator.
The published European patent application EP 0 823670 (assigned to Agfa-Gevaert) discloses an apparatus and method for applying a radiation curable composition on the image-side of a substrate bearing fused toner images. The means for applying the radiation curable composition can be rollers, wicks, sprays, screen printing, offset printing, and gravure rollers. In one embodiment, the means for fusing the toner particles and the means for curing the radiation curable composition are mounted in said apparatus directly adjacent to each other so that the curing proceeds on the warm curable composition.
Although known apparatus and processes may be suitable for their intended purposes, e.g., primarily to provide a protective overcoat on the toner images primarily for protection against mechanical interaction, they give unsatisfactory results with respect to the compensation for any possible topography in the toner images, the controllability of gloss, the gloss uniformity and the thickness control of the overcoat.